1. Field of Invention
The present invention relates to an illuminating optical system for virtually superimposing partial light beams onto the same illumination area after separating light emitted from a light source into partial light beams. In addition, the present invention relates to a projector which can display a uniform and bright image using the illuminating optical system.
2. Description of Related Art
In the projector, illuminating light shed upon an electro-optical device called a light valve is modulated in accordance with information of an image which one wants to display, and the modulated light is projected onto a screen to display the image. It is preferable that the image displayed by such a projector be uniform and bright, and that the efficiency with which light is used by the illuminating optical system applied to the projector be high.
As electro-optical devices, liquid crystal panels (liquid crystal light valves) of the type which modulate only one type of linearly polarized light are often used. When such liquid crystal panels are illuminated with unpolarized illuminating light, only one of the two types of linearly polarized light, perpendicular to each other, contained in the unpolarized illuminating light is used, so that the other type of linearly polarized light is not used. Therefore, when an illuminating optical system which emits unpolarized illuminating light is applied to a projector which uses the above-described liquid crystal panel, the efficiency with which illuminating light is used is reduced. To overcome this problem, there has hitherto been used an illuminating optical system using a polarization conversion optical system which converts unpolarized light emitted from the light source into one type of linearly polarized light.
Ordinarily, the intensity of light beams emitted from a light source is highest near the optical axis of the light source and tends to decrease as the distance from the optical axis increases. When such light beams are used as illuminating light beams, a non-uniform image is displayed at the projector. To overcome this problem, an integrator optical system has hitherto been used as an optical system for uniformly illuminating a liquid crystal panel acting as illumination area.
FIG. 9 is a schematic structural view of a conventional illuminating optical system. The illuminating optical system comprises a light source 4120, a first lens array 4130, a second lens array 4140, a polarization conversion optical system 4150, and a superimposing lens 4160. The two lens arrays 4130 and 4140, and the superimposing optical system (superimposing lens) 4160 form an integrator optical system.
The first lens array 4130 includes a plural number of small lenses 4132. The second lens array 4140 includes a plural number of small lenses 4142 in correspondence with the plural number of small lenses 4132 of the first lens array 4130.
The polarization conversion optical system 4150 comprises a plurality of sets of a polarization separation film 4152 and a reflective film 4154 disposed in the x-axis direction, with each polarization separation film 4152 being formed parallel to its associated reflective film 4154. The polarization separation films 4152 and the reflective films 4154 are tilted by a certain amount with respect to the xy plane. A .lambda./2 phase film 4156 is provided at the light-outgoing side of each polarization separation film 4152.
The substantially parallel light beams emitted from the light source 4120 are separated into a plural number of partial light beams by the plurality of small lenses 4132 of the first lens array 4130. By the light condensing action of the small lenses 4132 of the first lens array 4130, the separated partial light beams are condensed near the small lenses 4142 of the second lens array 4140 and the polarization separation films 4152 of the polarization conversion optical system 4150. Of the components of the condensed light incident upon the polarization separation films 4152, one of the types of linearly polarized light component (for example, the p-polarized light component) is transmitted through the polarization separation films 4152, while the other type of linearly polarized light component (for example, the s-polarized light component) is reflected by the polarization separation films 4152. The other type of linearly polarized light component reflected by the polarization separation films 4152 is reflected by the reflective films 4154 and falls upon the superimposing optical system 4160. On the other hand, the one type of linearly polarized light component transmitted through the polarization separation films 4152 is incident upon the .lambda./2 phase films 4156 and is converted into a linearly polarized light component which has the same polarization direction as the other type of linearly polarized light component in order to strike the superimposing optical system 4160. The plurality of partial light beams which have struck the superimposing optical system 4160 are each virtually superimposed on an illumination area 4180. This allows substantially one type of linearly polarized light to illuminate the illumination area 4180.
In the above-described conventional illuminating optical system, the substantially parallel partial light beams separated by the first lens array 4130 are condensed so that they are incident upon the polarization separation films 4152. As a result, the partial light beams incident upon the polarization separation films 4152 are spatially separated from each other. The reflective films 4154 are disposed at locations where the partial light beams from the second lens array 4140 do not fall directly thereupon, and reflect the linearly polarized light component reflected by the polarization separation films 4152. Accordingly, the unpolarized light emitted from the light source is separated into two types of linearly polarized light beams by the polarization separation films 4152 and the reflective films 4154.
If the light beams emitted from the light source 4120 are ideal parallel light beams, the partial light beams to be condensed near the polarization separation films 4152 are condensed virtually at one point. However, actual light sources are not point light sources, so that the light beams emitted from the light source 4120 are not completely parallel, causing the partial light beams to form images which are spread by a certain amount. In order to convert unpolarized light emitted from the light source 4120 into virtually one type of linearly polarized light more efficiently, most of each of the partial light beams is made to fall upon its associated polarization separation film 4152. Here, the second lens array 4140 and the polarization conversion optical system 4150 are disposed so that they are virtually in close contact with each other. Therefore, it can be said that the light-outgoing surface of the second lens array 4140 and the light-incoming surface of the polarization conversion optical system 4150 virtually coincide. Consequently, in order to cause most of each of the partial light beams to fall upon its associated polarization separation film 4152, the x-axis direction width of each polarization separation film 4152 and each reflective film 4154 can be made equal to or greater than the x-axis direction width of its associated image formed by condensing each of the partial light beams.
For a projection lens (projection optical system) used in the projector, there is an incident angle limit value (maximum value) allowing effective projection of incident light. The incident angle limit value is called the swallow angle. The swallow angle is made large by using a lens with a small f-number. However, using a lens with a small f-number increases, for example, the size and cost of the projector, so that it is preferable to use a lens with a large f-number as projection lens. In other words, in the projector, it is preferable that the incident angle of light which strikes the liquid crystal panel, the projection lens, or any other optical device disposed at the back side of the illuminating optical system be small. Therefore, ordinarily, the characteristics of each optical element are often determined based on the projection lens used.
When a decision is made as to the projection lens to be used, the swallow angle is determined. Since there is a limit as to the size of the entire projector, the distance between the illumination area 4180 and the superimposing optical system 4160 is limited to a value equal to or less than a certain value. Therefore, when an attempt is made to gather as much illuminating light as possible within the swallow angle of the projection lens, the limit value of the x-axis direction width of the polarization conversion optical system 4150 is determined. When the x-axis direction width of the polarization conversion optical system 4150 is set equal to or less than this limit value, the x-axis direction width of the polarization separation films 4152 and that of the reflective films 4154 may become considerably smaller than the x-axis direction width of the images formed by condensing light. In such a case, the unpolarized light emitted from the light source cannot be efficiently converted into virtually one type of linearly polarized light.